FR-A-93/00294, WO94/12055 and WO94/25575 describe a method for increasing the firmness and consistency of fruits or vegetables by the use of pectin methylesterase. FR-A-93/00294 describes the optional addition of from 0.5 to 2.0% of a calcium salt such as calcium carbonate. WO94/12055 describes the optional addition of calcium chloride to fruit or vegetable material which has been treated with pectin methylesterase and which is naturally low in calcium. WO94/25575 states that the firming of fruits and vegetables, and increasing the viscosity or producing gel formation in vegetable or fruit based products, can be accomplished by treating these components with pectin methylesterase and divalent cations, e.g. divalent ions such as calcium.
In "Journal of Food Science-Volume 57, No.5, 1992, pp.1144-1147", the article entitled "Optimization of a Diced Tomato Calcification Process" by John D. Floros et al states that processing of diced tomatoes at a CaCl.sub.2 concentration of 0.43% for 3.5 minutes would yield a product with increased firmness. Firmness is critical for diced tomatoes in aseptic processing and packaging, because the diced product is subjected to high shear stress as it is transferred through pumps, pipes, strainers, pressurized tanks and fillers. In diced pieces, the firmness has been attributed to the formation of calcium/pectate cross-links in the middle lamella and cell wall induced by the added calcium ions. These new calcium/pectate cross-links increase the rigidity of the tomato pieces and may also cause a decrease in the activity of the tomato polygalacturonase, thereby inhibiting the degradation of pectin in the middle lamella, cell wall and pericarp tissue in general by endogenous hydrolases such as polygalacturonase. During the CaCl.sub.2 treatment, the enzyme pectin methylesterase is believed to remain active, creating additional calcium/pectin cross-links and thus generating further rigidity in the tomato pieces.
It should be noted that in diced tomato pieces, the cellular material is only slightly disrupted whereas in macerated tomatoes used, for instance, for preparing a tomato paste, the cellular material is almost completely disrupted. In the normal industrial process for the manufacture of tomato paste, the tomato tissue is macerated and then immediately heat-treated to above about 82.degree. C. to inactivate the pectin degrading enzymes naturally present in the tomato. In the absence of this enzyme inactivation step, significant breakdown of the pectic and other carbohydrate polymers will result in a product of low consistency (see U.S. Pat. No. 4,547,375). This heat treatment to inactivate the pectin degrading enzymes naturally present in the tomato is known as the "hot-break" in the tomato industry. It is widely held in the tomato industry that the pectin degrading enzymes (pectin methylesterase, polygalacturonase, pectin lyase) and other cell wall carbohydrases such as the enzyme exo-(1-4) beta-D galactanase (WO95/232228) and the endobeta (1-4) glucanases (Lashbrook, et al. (1994) The Plant Cell 6, (1485-1493) will degrade the carbohydrate polymers responsible for high consistency if they are not immediately inactivated by heat. Therefore it is assumed that, to obtain tomato paste having a high consistency, the enzymes in the macerated tomatoes should be inactivated by heat as soon as possible by the "hot-break" method (see W.Gould "Tomato Production, Processing & Technology, 3.sup.rd Edition (1991) CTI Publications Inc).